Physical activity provides important health benefits to children and adolescents. Currently, however, physical activity levels decline significantly as children progress into adolescence, and few U.S. high school students meet current physical activity guidelines. The proposed study (TRACK-2) will extend an existing observational cohort study (TRACK-1) to identify salient factors that explain the changes in physical activity that occur as children transition from elementary school through middle school and into high school. Major goals of the study are to inform future physical activity interventions and to expand the body of knowledge regarding the relationship between physical activity and selected health indicators, including body fatness, during childhood and adolescence. The specific aims include 1)To identify personal, social environmental and physical environmental factors that influence the age-related trajectory for physical activity in youth as they transition from elementary school to high school; 2) To describe the longitudinal associations among physical activity, sedentary behavior and measures of body weight status in youth as they transition from elementary school to high school; and 3) To describe the relationship between physical activity, observed longitudinally during childhood and adolescence, and selected health indicators in high school students. The study will measure in approximately 850 9th and 11th grade students, as well as their parents, schools and communities, all of the variables that were measured in the same students in 5th, 6th and 7th grades, including physical activity measured by accelerometry. Additional variables that are relevant for high school students also will be measured. Three of these variables - sleep, fitness and academic achievement - will be measured in a sub- sample of the cohort in 10th grade. The proposed research is significant because it will identify the factors that underlie age-related changes in physical activity, providing knowledge needed to design effective public health interventions to increase physical activity and prevent obesity in American children and youth. The study is innovative because it will be the first to follow a sizeable and diverse cohort of American youth as it transitions from elementary school through middle school and into high school and will use multi-level modeling to examine a comprehensive and carefully-measured set of potential predictors of change in physical activity.